CO2 sensing characteristics of Sm1-xBa CoO3 (x = 0, 0.1, 0 ...s2is.org/Issues/v1/n3/papers/paper3.pdf · nanostructered powders were characterized by X-ray diffraction ... of sintered

$Page 1: CO2 sensing characteristics of Sm1-xBa CoO3 (x = 0, 0.1, 0 ...s2is.org/Issues/v1/n3/papers/paper3.pdf · nanostructered powders were characterized by X-ray diffraction ... of sintered$
CO2 sensing characteristics of Sm1-xBaxCoO3 (x = 0, 0.1, 0.15,

0.2) nanostructured thick film

G.N. Chaudhari, P.R. Padole, S.V. Jagatap, M.J. Pawar*

Nanotechnology Research Laboratory, Dept. of Chemistry,

Shri Shivaji Science College, Amravati, M.S. India.

[email protected]

Abstract:

SmCoO3 gas sensor has been developed with high sensitivity for CO2 gas by doping Ba.

Nanostructured SmCoO3 and Sm1-xBaxCoO3 (x = 0, 0.1, 0.15, 0.2) were obtained by EDTA-

Glycol method. The operation temperature falls and sensitivity increases from 425 to 370OC

when Ba concentration in SmCoO3 changes from x = 0 to x = 0.1. Ag impregnation over

Sm0.9Ba0.1CoO3 sensor, on exposure to CO2 at about 360OC showed an increased sensitivity as

well as the response time also decreases. The possible CO2 sensing mechanism is proposed on

the basis of available literatures.

Keywords: SmCoO3; Nanostructures; Gas Sensitivity; Selectivity; EDTA-Glycol Method;

1. INTRODUCTION

Growing environmental awareness and hazards of global warming has increased the need

for CO2 measurements in many countries. CO2 is one of the most common gases evolved

in the combustion; it is too stable chemically to be detected in a sensitive manner by

conventional gas sensors. The advanced technologies, such as air conditioning,

agriculture, biological technology and medical services have made the control and

measurements of CO2 concentration critical. Furthermore, the CO2 concentrations in the

atmosphere have increased on the global scale. Therefore, non-expensive and robust

detection systems are required for increasing carbon dioxide concentrations.

Unlike, optical and electrochemical sensors, solid state gas sensors based on

semiconductor metal oxides (SMOs) may be a promising alternative, since they offer

good sensor properties and can be easily mass-produced. Several SMOs viz. La-doped

SnO2 [1, 2], BaTiO3 [3, 4], etc are reported to the most reliable CO2 sensors. Among the

chemical sensors, LaCoO3, BaTiO3, LaFeO3, LaMnO3 etc are perovskite-type materials

of general formula ABO3 are extensively studied owing to their notable gas sensitivity

for different poisonous gases in addition to their magnetic, catalytic and other physical

properties. The perovskite-type metal oxide including the d-block and rare earth elements

INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 1, NO. 3, SEPTEMBER 2008

613

has attracted the attention of many researchers due to their homogeneity, interesting

structural, catalytic and gas sensing properties. Rare earth-cobaltites (LnCoO3; Ln =

Metals like La, Sm, Gd and Nd etc.) with perovskite-type structure exhibit outstanding

transport properties and high chemical activity, which make these materials suitable for

applications in areas of gas sensors, heterogeneous catalysis, gas separation membranes

and cathodes for solid oxide fuel cells. The materials of LnCoO3 type have been explored

for many technical applications technical applications, e.g. as electrodes in fuel cells [5]

and as auto-exhaust conversion catalysts [6, 7].

SmCoO3 material was studied as a catalyst for oxidation of CO in early eighties

[8].Sensing properties of Ba doped SmCoO3 has been studied at 410OC [9]. Many

approaches have been made to modify the sensing properties of these perovskite

materials in order to achieve high sensitivity, selectivity and to reduce the operation

temperature. In this direction different approaches are adopted including new methods of

material synthesis, doping suitable metals, physical or chemical filters etc. The purpose

of this work is to investigate and develop a new synthesis route for pure SmCoO3 and Ba

doped SmCoO3 as well as to study the effect of noble metals impregnation on gas

sensitivity of Sm1-xBaxCoO3.

Chemical sensors made of nanomaterials have great potential for an enhanced generation

of sensing devices that are smaller, consume less power, are higher-performing, and less

expensive than conventional sensors. Therefore, nanostuctured and single-phase

perovskites, Sm1−xBaxCoO3 (x = 0, 0.1, 0.15, 0.2), were prepared by an EDTA-glycol

method using stoichiometric amounts of the corresponding nitrates, ethylene glycol (EG)

and disodium salt of ethylene diamine tetra acetic acid (EDTA). So formed

nanostructered powders were characterized by X-ray diffraction and scanning electron

microscopy (SEM). XRD studies showed that the introduction of barium reduced the

temperature of formation of the perovskite and yielded nanostructured Sm0.9Ba0.1CoO3.

The evaluation of nanostructured Sm0.9Ba0.1CoO3 as a CO2 sensor was made through the

electrical characterization of sintered thick films.

2. EXPERIMENTAL

2.1 MATERIAL SYNTHESIS

The nanostructured compounds of Sm1-xBaxCoO3 were prepared for x = 0, 0.1, 0.15 and

0.2 by EDTA-glycol method. A calculated quantity of EDTA was firstly dissolved in a

small quantity of deionized water followed by the addition of stoichiometric ratio of

G.N. CHAUDHARI, P.R. PADOLE, S.V. JAGATAP, M.J. PAWAR, CO2 SENSING CHARACTERISTICS OF SM1-XBAXCOO3 (X = 0, 0.1, 0.15, 0.2) NANOSTRUCTURED THICK FILM

614

nitrates Sm, Ba and Co. The mixture was magnetically stirred at 60OC for 3 hr in order to

obtain stable metal-EDTA complexes. After stirring, appropriate amount of ethylene

glycol (EG) was added to this solution. The solution so obtained was continuously stirred

on a hot plate with a magnetic stirrer at 90OC for 7 hr to remove the excess of water and

facilitate the polyesterification between EDTA and EG leading into formation of a resin

like mass. Heating at 350OC the polymeric resin is decomposed. The decomposed resin

was treated in a mantle heater at 400-450OC over 3 hr in order evaporate highly

combustible species and induce charring. The resulting ash were slightly ground in to

powder and subjected to calcination at 750OC for 6 hr as summarized in Table 1.

In order to enhance the sensitivity for CO2 gas, sample GIV was further loaded with 0.5

wt. % of Ag and Pd. Appropriate weights of silver nitrate and palladium nitrate were

dissolved separately in distilled water and then a calculated quantity of sample GIV was

added to this solution. These solutions were then stirred on magnetic stirrer to maintain

uniform ness. After several hours, the samples were slowly heated to dryness with

stirring and then heated at 4000C for 6 hr in air. The samples at the final were finely

grounded into powders and represented as GIV: Ag and GIV: Pd.

Sm1-xBaxCoO3 Calcination

Temperature (OC)

Calcination Time

(Hr) Sample Code

SmCoO3

Sm0.9Ba0.1CoO3

Sm0.85Ba0.15CoO3

Sm0.8Ba0.2CoO3

750

750

750

750

6

6

6

6

GI

GII

GIII

GIV

Table 1. Sensor samples with experimental conditions.

The samples were characterized by X-ray diffraction (XRD) analysis and scanning

electron microscopy (SEM). In the present article, results are discussed only for SEM

analysis.

2.2 GAS SENSING MEASUREMENTS

The sensitivity for CO2 gas was measured for all the samples by preparing their thick-

films. The sensor materials were deposited through screen-printing technique on the

alumina substrate. Two Platinum electrodes separated by distance of 2 mm were printed

on the surface of alumina tube for the measurement of gas sensing properties. The paste


615

was prepared by mixing sensor samples with 30 wt. % organic binder, 30 wt. % solvent

and 0.15 ml. dispersant. Thick-films, after screen-printing on alumina substrate were

kept at 2000C for 2 hr for removal of the organic binder. The thick-film devices must be

heated to sufficiently high temperatures to ensure interconnectivity between individual

grains and film robustness. To measure gas sensitivity, the sensors were placed in test

chamber through which dry air and gas to be sensed were allowed to flow. The operating

temperature was measured by use of a thermocouple kept touched to the sensor system.

Before taking the measurement the sensor was heated to 3500C, cooled in dry air since

these sensors are influenced by humidity in atmosphere. Finally, the sensitivity of thick-

films was measured for CO2 gas for the temperature starting from 150 to 450 0C. The

sensitivity (S) to CO2 was defined as the ratio of resistance of an element in CO2 free air

(RAIR) to the resistance in a CO2 gas (RCO2), RAIR / RCO2. The change in resistance on

contact between target gas and sensor was recorded from multimeter.

3. RESULT AND DISCUSSION:

3.1 CHARACTERIZATION

From the XRD analysis, carried out on calcinated doped powders of SmCoO3, only the

signals corresponding to the oxides of Sm, Ba and Co were observed. No secondary

phase was observed in these samples though they are calcined at lower temperature;

750OC. These observations indicate a better mixing of the cations in the compounds

formed. In order to obtain the insight information about the surface morphology and

particle size of the sample, SEM analysis was performed. Fig.1 shows the SEM

micrograph of the sample GII. The formation of Sm0.9Ba0.1CoO3 aggregates comprising

very tiny three-dimensional disordered nanoparticles was visibly observed. The particle

size of the sample GII is quite uniform of about 60 nm.

Figure1. SEM photomicrograph of the Sm0.9Ba0.1CoO3


616

3.2 SENSITIVITY MEASUREMENT OF NANOSTRUCTERED Sm1-XBaXCoO3

Fig. 2 shows the gas sensitivity for 1000 ppm CO2 gas for samples GI, GII, GIII and GIV

at different temperatures ranging from 150 to 450OC. The maximum sensitivity was

observed at about 425OC for undoped sample GI, which reduced to 370OC for sample

GII. The gas sensitivity increased as compared to GI on Ba addition. It could be due to

uniform distribution of dopant at the molecular level and the reduction of particle size on

Ba addition (for x=0.1). From the Fig.2 it can be seen that, for samples GIII and GIV no

significant rise in sensitivity though the concentration of Ba is increased from x = 0.15 to

0.2. Maximum gas sensitivity increased from about 0.35 to 0.87 from samples GI to GII

at 370OC. Even without the noble metal catalyst additions the gas sensitivity

enhancement in this case was about four times.

Fig. 3 shows the sensitivity at 370OC to CO2 as a function of the amount of Ba

dopants. The sensitivity of the element increases substantially with increasing amount of

Ba up to x = 0.1. However, the sensitivity is maintained in the range from x = 0.1 to x =

0.15 and further addition (x = 0.2) decrease the sensitivity for CO2. It seems that x = 0.1

is an enough amount of Ba as a dopant for increasing the sensitivity of SmCoO3.

1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 00 .0

0 .2

0 .4

0 .6

0 .8

1 .0

O p e r a t io n T e m p e r a tu r e ( O C )

Sens

itivi

ty (S

)

S a m p le G I S a m p le G II S a m p le G II I

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0 S a m p le G IV

Fig. 2 CO2 sensing characteristics of the samples GI, GII, GIII and GIV as a function of operation

temperature (OC).

0 .0 0 .1 0 .20 .2

0 .4

0 .6

0 .8

1 .0

X in S m1 -x

B axC o O

3

Sens

itivi

ty (S

)

0 .2

0 .4

0 .6

0 .8

1 .0

Figure 3 CO2 sensing characteristics of SmCoO3 as a function of doped Ba at 370OC


617

3.3 EFFECT OF Ag AND Pd ADDITIVES ON CO2 DETECTION

The noble metals, well known as active catalysts, have been confirmed to possess the

promoting effects on many semiconductor gas sensors [10–12]. Doping with the noble

metals such as Pt, Pd, Ag and Au can raise the adsorption activity of the semiconducting

oxides. These metals form cluster on the oxide surface, promoting adsorption and

dissociation of reducing gases [13-16], and thus are well known for enhancing the rate of

response with decreased operation temperature and raising selectivity to single gas. In

the present study, we have tested the effect of 0.5 wt. % Ag and Pd impregnation over

the sample GII as. The sample GII was only selected for the further study as it has shown

the higher sensor response for CO2 among all the samples.

150 200 250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

Operation Temperature (OC)

Sens

itivi

ty (S

)

Sample GII:Ag

0.0

0.2

0.4

0.6

0.8

1.0

Sample GII:Pd

Figure 4 CO2 sensing characteristics of Ag and Pd impregnated sample GII as a function of operation

temperature (OC)

Fig. 4 depicts the gas sensitivity of sample GII: Ag and GII: Pd as a function of operating

temperature. One can see from the fig. 4 that, the sensitivity to CO2 gas of sample GII

with 0.5 wt. % Ag is about twice as large as that for GII: Pd at about 360OC. The

addition of Ag gives a high sensitivity to CO2, but the operating temperature is not much

reduced. The high operating temperature is not desirable for practical applications and

long term stability. The mechanism for gas detection in these materials is based, in large

part, on reactions that occur at the sensor surface, resulting in a change in the

concentration of adsorbed oxygen. The enhancement of sensing property seems to be

brought about by changes in the surface property caused due to impregnation of Ag.

Since the amount of adsorbed CO2 increased greatly by the addition of Ag, as the added

Ag catalyses the surface carbonation of the sensor element.


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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 340.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Sample GII

Sample GII:Ag

Time (min.)

Sens

itivi

ty (S

)

0.0

0.2

0.4

0.6

0.8

1.0

Figure 5 Response characteristics of sample GII (at 370OC) and sample GII:Ag (at 360OC)

100 200 300 400 500 600 700 800 900 10000.2

0.4

0.6

0.8

1.0

CO2 Gas Concentration (ppm)

Sens

itivi

ty (S

)

0.2

0.4

0.6

0.8

1.0

Figure 6 Sensing characteristics of sample GII: Ag as a function of CO2 concentration (ppm) at 360OC

In case of GII: Pd, a monotonic rise in sensitivity can be seen up to 200OC and beyond

this temperature, sensitivity falls. Though the PdO is a well known good catalyst for

chemical sensors but is not very active beyond 200OC may be because of its

decomposition at 200OC.

The change in sensitivity of GII: Ag element with the concentration of CO2 at 360OC is

shown in Fig. 5. The sensitivity monotonically increased with the increase in

concentrations of CO2 from 100 to 1200 ppm. Fig. 6 shows the response characteristics


619

of the samples GII and GII: Ag. The Ag impregnated GII showed a high sensitivity as

compared to the undoped element.

For the CO2 sensors, selectivity is an important subject, and in particular, removing the

interference of from humidity is critical. To check the selectivity, the effect of 1.5 vol. %

water vapors on the gas sensitivity of sample GII was investigated at 360OC. The

presence of water vapors decreased the sensitivity to CO2 by the factor of about 5.4. The

similar types of results were also reported by Tamaki et al. [17]. The addition of Ag to

GII did not remove the interference of humidity in CO2 sensing.

3.5 SENSOR STABILITY

In practice, the stability of the sensors is one of the most important parameters in the

sensor technology. In order to check the long-term stability of the noble metal loaded

sensors, they were tested for 1000 ppm of CO2 after the period of four months at the

optimum temperature of both the samples. The results obtained in this measurement are

shown in Figure 7. It was observed that, the Ag loaded sample losses its sensitivity by

±20%. On the other hand, undoped sample stands with high stability and maximum loss

in sensitivity was not more than 15%.

50 100 150 200 2500.0

0.2

0.4

0.6

0.8

1.0

Sample GII:AgSample GIISamples

Sens

itivi

ty (S

)

0.0

0.2

0.4

0.6

0.8

1.0At 360OC

At 370OC

Figure 7 Sensitivity of sample GII (at 370OC) and GII:Ag (at 360OC) towards 1000 ppm of CO2 after the

time period of four months

3.4 SENSING MECHANISM

The sensing mechanism of SmCoO3 thick-film gas sensors is usually based on surface

properties of material. The molecular oxygen in the air gets adsorbed on the surface of

sensing material and dissociates to produce different species like O, O2- and O-. There is

a formation of a depletion layer among the adsorbed oxygen ions and the oxide particles


620

caused due to transfer of conduction electron. This develops an electric field between

adsorbed oxygen species and the oxide surface. At the specific temperature CO2 can be

oxidized in to CO3- by interacting with adsorbed oxygen species. These carbonates

disappear when they are exposed to oxidizing conditions [18]. When the sensor response

measurement is carried out at higher temperature, the desorption of gas leads into the

sensitivity of material.

O2 + e-→ O2- (1)

O2 + 2e- → 2O2- (2)

CO2 + O- → CO3- (3)

4. CONCLUSIONS

In conclusion, the CO2 sensing property of SmCoO3 was improved by doping Ba. The

operation temperature was decreased from 425OC (for sample GI) to 370OC (for sample

GII). The sample GII (Sm0.9Ba0.1CoO3) impregnated with Ag showed rather high

sensitivity and stability with quick response at the reduced temperature of 360OC. A big

fall in the sensitivity for CO2 gas in presence of humidity is a major disadvantage of the

sample GII: Ag, which needs further improvement.

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CO2 sensing characteristics of Sm1-xBa CoO3 (x = 0, 0.1, 0 ...s2is.org/Issues/v1/n3/papers/paper3.pdf · nanostructered powders were characterized by X-ray diffraction ... of sintered